1. Field of the Invention
The present invention relates generally to tanks in a fluid, and in particular to an improved construction for a tanker vessel for transporting liquid cargoes.
2. Description of the Prior Art
Tanker vessels for the transportation in bulk of liquid cargo are known in the art. See, for example, U.S. Pat. No. 2,918,032. Such tanker vessels generally comprise a plurality of liquid-tight transverse bulkheads and one or more liquid-tight longitudinal bulkheads which subdivide the tanker vessel into a plurality of liquid-tight storage compartments. If the bottom or a side of the hull of such a tanker vessel is ruptured by grounding or some other accident, the affected cargo tanks will leak until the "pressure head" of the liquid cargo in each tank, i.e., approximately the portion of the liquid cargo disposed above the waterline of the vessel, flows out of the tanker vessel. Such leakage is a significant potential problem in so-called "SWBT" and "double-bottom" tanker vessels which have greater freeboard than conventional tanker vessels, and, hence, a greater cargo pressure head.
In recent years, pollution by oil tankers as a result of a hull rupture caused by grounding or other major catastrophe has become of increasing concern. As a result, various anti-pollution tanker constructions have been proposed. One of these is the so-called "double-bottom" tanker design which essentially comprises a tanker vessel having two spaced-apart hull bottoms. The purpose of this design is to prevent leakage from the tanker's cargo tanks if the outer hull bottom is ruptured by, for example, grounding. Such a design may not prevent leakage, however, where major damage is caused by grounding or some other accident since such damage may also cause the inner hull bottom to rupture in addition to the outer one. Moreover, besides the additional expenses involved in manufacturing such a tanker, the space between the inner and outer hull bottoms is unusable for the transportation and storage of cargo and, as a result, such a design increases the expenses of operating and maintaining the tanker. Bottom damage repair costs are also significantly greater in such tanker vessels, and such double-bottom tanker vessels require a rigid internal structure to support the huge loading stress of the cargo on the inner hull bottom and water on the outer hull bottom which tends to rupture both the inner and outer hull bottoms when hull damage occurs.
It has also been proposed to construct a tanker vessel with double sides defining side tanks extending from the top deck of the vessel to the hull bottom which are disposed adjacent to and associated with cargo tanks in the vessel. See U.S. Pat. No. 3,832,966. These side tanks have a volume from the hull bottom to the waterline of the tanker vessel which is equal to the respective volumes of the cargo tanks above the waterline. Valves coupling the side tanks to the cargo tanks are opened if the tanker hull is ruptured to permit oil in the cargo tanks to drain off into the side tanks so that the oil above the waterline in the cargo tanks does not escape from the ruptured hull bottom. Larger vents are used in the side tanks than in the cargo tanks to achieve this drainage from the cargo tank to the side tanks instead of into the sea. The disadvantage of this design, however, is that the side tanks provided for receiving the "pressure head" of the liquid cargo carried in the cargo tank is, similar to the space between the inner and outer hull bottoms in a double-bottom tanker, unavailable for the storage and transportation of cargo and, accordingly, increases the fabrication, maintenance and operating costs of the tanker vessel. Moreover, such a design is theoretical only and in practice would save no more than 2 or 3% of the liquid cargo carried by such a tanker vessel.
Generally speaking, ocean-going tanker vessels are required by international regulations to have a minimum amount of extra buoyancy to provide for floatation in the event of a hull rupture due to grounding, collision, or the like. This extra buoyancy is controlled by an assigned vessel free-board which is determined by measurements and calculations for each vessel. Existing ocean-going vessels generally have free-board assignments which, depending upon their size, place the main deck of the vessel at a location which is approximately 11 to 23 feet or more above the waterline of the vessel when fully loaded. Thus, since petroleum products, with rare exceptions, are lighter than water, in the event of a rupture below the waterline the products are supportable by water only to a predetermined level above which any cargo located in the cargo compartment will displace an equivalent amount of cargo through the rupture in the hull.